This invention relates to stencil screen printing, and more particularly to curvalinear stencil screen printing apparatus particularly suited for repeat printing on a web.
Ability to stencil print repeat patterns on a generally continuous web enables total printing cost per item to be significantly reduced and enables production output to be increased. Equipment to effectively achieve stencil printing on web stock is set fourth in U.S. Pat. No. 3,779,160. Such equipment reciprocates the stencil and its frame over the print surface an amount slightly greater than twice the length of the area to be printed. Consequently, the practical size of the area to be printed is limited. Above a certain size, the large reciprocating stencil frame necessary would require too much valuable space for its reciprocal movement. Also, the large heavy stencil frame becomes unwieldy to reciprocate. And, if the structure were strengthened to support it, still the speed of printing would be lessened due to the necessity to more slowly reciprocate the heavier stencil frame. The heavier the frame, the heavier is the driving and braking equipment required to repeatedly accelerate it, stop it, accelerate it in the opposite direction, stop it, and so forth. Therefore, to print large items is normally not practical or, if done, involves printing of segments thereof to be subsequently assembled, e.g. as with large posters.
Achieving high quality when using very large stencils on a flat bed press presents other problems. The large stencil tends to sag at the center, so that, after the print stroke, the separation of the stencil screen from the stock too frequently is not a neat separation, causing distortion of some freshly printed areas. And the ink tends to puddle at the center of the screen.